In certain methods of treatment of spinal disorders it is desirable to substantially immobilise selected segments of the spinal column against movement relative to one another. For example, one implant system has involved the use of screws, nuts and plates, the screws being inserted in pedicles of selected segments of a spinal column and the plates attached to the screws by means of nuts. Such a system has a number of disadvantages amongst which are difficulties experienced in bending the plate or plates to the required curvature to fit. In another system plates are placed in position first and are then secured by screws and there is considerable risk of breakage of the screws as a result of movement. In yet another system screw threaded rods are used instead of plates or smooth rods and are fixed to slotted pedicle screws by nuts with one nut at each side of each screw, but considerable difficulty is experienced in fixing the rods in the pedicle screws by the nuts as a consequence of the contour of the lumbar spine. In another implant system, slot headed screws have been used in cooperation with rods clamped in the slots of the screws by securing screws; the clamping of the rods has not always been satisfactory and the process of screwing the securing screws into the heads of the slot headed screws has created a substantial risk of splitting the slotted heads o the screws and also has made it extremely difficult to maintain a desired degree of tightness of clamping since the act of tightening one securing screw creates the risk of loosening of an adjacent securing screw.